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Glutamate receptorsThe cellular localization of NMDA and AMPA type glutamate receptor subunits was studied in inner ear. In the spiral and vestibular ganglion, almost all cells were immunoreactive for the NMDARl subunit and the AMPA type receptor subunit GluR2/3 and 4. This indicates that both NMDA and non-NMDA type glutamate receptors may be co-distributed in the primary afferent neuronal components, and are possibly involved in neurotransmission in the primary auditory and vestibular systems.Glutamate transporterGlutamine synthetaseConfocal microscopy revealed that GLAST and glutamine synthetase were colocalized in supporting cells apposed to the immunonegative hair cells. Postembedding immunoelectron microscopy revealed that GLAST was heterogeneously distributed along the plasma membranes of the supporting cells, with higher concentrations basally (at the level of the afferent synapses) than apically. The present findings suggest that glutamate released at the afferent synapse of vestibular hair cells may be taken up by adjacent supporting cells and converted into glutamine. Thus, at this peripheral synapse, the supporting cells may carry out functions similar to those of glial cells in the CNS.